Optical filters have been designed that attenuate sufficiently at a particular wavelength thus leaving the rest of the visible spectrum open. Since the use of filters greater than optical density-2 (OD-2) limits the performance of pilot/operator in bright sunlight conditions, it follows that the use of OD-2 filters in a low ambient light scenario would likewise limit the performance for non-instrumental surveillance at the latter conditions. Bright light in the visible region (e.g., wavelength from about 397 micrometers or microns to about 723 micrometers or microns) and to the infrared region to the limit of perception of human eye (which is about 100 microns or 100 micrometers), emitting from a point source relates to radiation of a different nature than that which is emitting from a wide spectrum electromagnetic radiation source extending in many directions. Radiation from a point source such as radiation from a frequency agile laser is the subject matter which is covered in greater detail hereinbelow.
Laser radiation particularly in the visible region has always presented a potential hazard to the human eye. More recently this potential has been under exploitation by military organizations both foreign and domestic. The human eye focuses light in the visible region which greatly increases the intensity of light on the retina from that striking the cornea and the lens.
Certain laser beams are 10,000 times brighter than the sun's rays, and much more hazardous. When a beam of laser radiation is absorbed by living tissue, the extent of damage caused is dependent on several things: the energy level of the radiation, the type of tissue irradiated, and the wavelength of the laser radiation, and the time of exposure to the radiation.
Intense laser-energy when absorbed by the body is converted into heat. This heat coagulates the protein in the body's tissues (in a similar manner when boiling water coagulates egg albumin) and destroys the cells.
The human eye is the most vulnerable tissue to all types of laser radiation. The tissue in the retina (that portion of the eye upon which the light or image is focused, and specifically the fovea of the retina) is particularly susceptible to damage because the lens of the eyeball concentrates and focuses the laser beam on the fovea of the retina.
The interaction of a laser beam with eyeball tissue as received by the cornea and focused by the lens on the fovea of the retina is described as follows:
1. As the laser beam impinges on the eyeball, part of the beam is prevented from entering the eye by the iris, a colored disc behind the cornea (the outwardly convex transparent membrane forming part of the anterior outer coat of the eye); the iris acts like an automatic photographic shutter and constricts when high-intensity light impinges on the eye;
2. The shutter action of the iris prevents part of the light from reaching the retina;
3. The part of light which reaches the retina also affects a thin, dark-brown choroid membrane containing arteries, veins, and pigment cells which surrounds the retina. This membrane being dark colored can easily absorb this harmful radiation; hence, it must be protected;
4. The laser beam is converged and focused on the fovea of the retina by the lens; and,
5. As projected from laser-welding principles we know that the focus is the hottest point, therefore, the laser-energy density at the fovea (which is the focal point) is about 10.sup.4 to 10.sup.6 times more concentrated than that received by the cornea and the lens.
As concluded from the above described interactions, it is recognized why eye damage can occur due to this magnitude of light concentration.
Since the laser when used as a weapon against the human eye, eyes behind optics, and optics themselves is considered a threat to U.S. Forces, scientists have looked hard at contermeasure techniques. Protective goggles or glasses have been the only real product of research efforts. The bulk of the work has been in the types of filters to go in this eyewear.
Filters have been designed that attenuate sufficiently at a particular wavelength thus leaving the rest of the visible spectrum open. This is a real advantage to the pilots or operators in performing their tasks. With the advent of frequency agile lasers however, it has become necessary to filter over the entire visible region. This presents a real handicap to the operators in performing their tasks particularly in a low ambient light scenario.
The use of an optical gate or shutter to block out the entire visible region has been considered. The problem with this conceptive idea is that there is just not enough time to detect radiation and actuate a gate or shutter before damage is done by a first laser pulse. The gate or shutter could be dropped over the eyes of the pilot or over a particular part of the cockpit itself. The latter type of protection is attractive for use as a shield from the intense flash in a nuclear explosion since it employs the electro-magnetic pulse as a source for detection far enough in advance of the flash of a nuclear explosion so that time permits the dropping of a shield in advance of the arrival of the flash radiation. In the case of a laser pulse there is not sufficient time to detect a first laser pulse and actuate a shutter/gate before damage is done to the eye.
An object of this invention is to take advantage of this shutter/gate idea by employing radiation detecting devices for accepting the first pulse, processing it, and initiating activating procedures which protect against subsequent pulses. The probabilities here are in favor of the operator since the chance that the operator will not be looking directly at the source of the laser upon the first pulse or that the pulse will be off target (assuming operator to be target) but is still detectable. The described protection for a second pulse based on the probability that the greatest danger may be that a operator will instinctively look towards the source when the second pulse arrives. The plan of action is to prevent this instructive look toward the radiation source from taking place by detecting the first pulse and then triggering the shutter to protect against subsequent pulses.
Therefore, a further object of this invention is to provide a detection device for a first laser pulse which is subsequently processed and employed in combination with a shutter triggering control for dropping a protective shutter against agile laser radiation.